When Attempting to Predict or Model the Future Benefits That

When attempting to predict or model the future benefits that might accrue from an effective preventive intervention, should the reference category for the causal agent
Of interest, e.g.sun exposure or cigarettes, (i.e.the level of exposure to which the
Intervention aims to shift the population) be:

A) The null category, i.e.zero exposure to the cause
B) Half the current exposure level
C) A plausible minimum level of the degree of change that seems achievable based on evidence from trials or other sources
D) The average exposure in a different population with lower disease rates (e.g.for an Australian population, the reference level could be sun exposure in England)
E) A, C and D